An optical system having a compact coupling from a projector into a waveguide

Abstract

An optical system, comprising: a light guiding optical element (LOE) (10) having first and second main outer surfaces (11, 12) parallel to each other for guiding light by total internal reflection; and a projector (100) projecting illumination corresponding to a collimated image from an aperture (101). The projector injects light into the LOE via a coupling prism (30) attached to the first main outer surface (11), which projects an image injection surface. A reflective polarization beam splitter (51) is deployed at the interface between the main outer surface (11) and the coupling prism (30) parallel to the main outer surface to selectively transmit the illumination from the coupling prism into the LOE while capturing the light that has already been within the LOE in order to propagate within the LOE by total internal reflection.

Description

Technical Field

[0001] The present invention relates to an optical system and, in particular, to an optical system for compactly coupling an image from a projector into a waveguide. Background Art

[0002] Many virtual reality and augmented reality displays employ a light-guide optical element (LOE) having two main parallel flat surfaces, within which an image propagates by internal reflection. Illumination corresponding to a collimated image is generated by a projector and introduced into the LOE in a coupling region. The illumination propagates within the LOE by internal reflection until it reaches a coupling-out region, where the illumination is coupled out of the LOE towards the observer's eye. Coupling out the illumination towards the eye can be performed by using a set of partially reflective inner surfaces at an inclined angle, or by using one or more diffractive optical elements, all of which are well known in the art. Coupling the image illumination from the projector into the LOE can be achieved via a coupling prism. Summary of the Invention

[0003] The present invention is an optical system for compactly coupling an image from a projector into a waveguide.

[0004] According to the teachings of an embodiment of the present invention, there is provided an optical system comprising: (a) a light-guide optical element (LOE) formed of a transparent material and having a first main outer surface and a second main outer surface that are parallel to each other for guiding light by internal reflection; (b) a projector configured to project illumination corresponding to a collimated image from an aperture, the illumination leaving the aperture along a chief ray that defines the optical axis of the projector and within an angular field around the chief ray; (c) a coupling prism attached to the first main outer surface of the LOE, the coupling prism providing at least a portion of an image injection surface that is inclined with respect to the main outer surface, the projector being associated with the image injection surface and oriented such that the chief ray and the angular field around the chief ray are injected through the image injection surface at an incident angle with respect to the main outer surface, the incident angle being greater than the critical angle of internal reflection at the main outer surface; and (d) a reflective polarization beam splitter disposed at an interface between the main outer surface and the coupling prism parallel to the main outer surface, at least a portion of the illumination being incident on the beam splitter with a first polarization, and being transmitted by the beam splitter from the coupling prism into the LOE, light corresponding to a conjugate image of the collimated image and having a second polarization incident on the beam splitter from within the LOE being reflected by the beam splitter so as to propagate within the LOE by internal reflection.

[0005] According to another feature of an embodiment of the present invention, a wave plate is further provided and disposed in the path of at least a portion of the illumination to convert the illumination between a first polarization and a second polarization.

[0006] According to another feature of an embodiment of the present invention, the wave plate is a quarter-wave plate associated with at least a portion of the second major outer surface of the LOE.

[0007] According to another feature of an embodiment of the present invention, the wave plate is a half-wave plate that is deployed to overlap a first portion of the aperture and not overlap a second portion of the aperture.

[0008] According to another feature of an embodiment of the present invention, the first portion of the aperture projects illumination through the following portion of the image injection surface: light enters the LOE from the said portion of the image injection surface without passing through the beam splitter.

[0009] According to another feature of an embodiment of the present invention, the projector is configured to project illumination of a second polarization, wherein the first portion of the aperture projects illumination through the following portion of the image injection surface: light passes through the beam splitter from the said portion of the image injection surface, and the half-wave plate converts the illumination of the second polarization into illumination of the first polarization.

[0010] According to another feature of an embodiment of the present invention, the image injection surface is provided partly by a coupling prism and partly by the surface of the LOE.

[0011] According to another feature of an embodiment of the present invention, the image injection surface is provided entirely by a coupling prism. Description of the Drawings

[0012] The present invention is described herein by way of example only with reference to the drawings, in which:

[0013] Figure 1 is a schematic side view of a projector injecting an image into a waveguide via an edge surface, which indicates an incomplete filling of the waveguide with the image;

[0014] Figure 2A and Figure 2B is a schematic side view of a projector injecting an image into a waveguide via a coupling prism to fill the thickness of the waveguide with the image, which shows the gentle extreme and the steepest extreme of the projected angular field, respectively;

[0015] Figures 3A to 3C is a schematic side view of an optical system according to an embodiment of the present invention, the optical system including a projector injecting an image into a waveguide via a coupling prism having a reflective polarization beam splitter, which shows the gentle extreme, the intermediate field, and the steepest extreme of the projected angular field, respectively;

[0016] Figure 4 is a graph showing the transmittance of p-polarization and the reflectance of s-polarization as a function of the incident angle of a polarization beam splitter suitable for use in an embodiment of the present invention;

[0017] Figure 5A and Figure 5B is a schematic side view of an optical system according to a variant embodiment of the present invention, which shows respectively the gentle extreme and the steepest extreme of the projected angular field;

[0018] Figure 6A and Figure 6B is a schematic side view of an optical system according to another variant embodiment of the present invention, which shows respectively the gentle extreme and the steepest extreme of the projected angular field;

[0019] Figures 7A to 7C is a schematic side view of an optical system according to any of the above embodiments, which shows a possible deployment of a retarder element for reducing polarization-related banding effects; and

[0020] Figure 8A and Figure 8B is a schematic side view of an optical system according to any of the above embodiments, which shows respectively the possible deployment of one or two internal partial reflectors to achieve mixing for reducing polarization-related banding effects. DETAILED DESCRIPTION

[0021] The present invention is an optical system for compactly coupling an image from a projector into a waveguide.

[0022] The principles and operation of the optical system according to the present invention can be better understood with reference to the accompanying drawings and the following description.

[0023] By way of introduction, Figure 1 shows light rays propagating within a light guide optical element (LOE) 10 (interchangeably referred to herein as a "waveguide") by internal reflection at mutually parallel first and second major outer surfaces 11 and 12. In this example, the light rays are coupled out towards the observer's eye 40 by an embedded partial mirror 20 that is inclined with respect to the major outer surface of the LOE. As is well known in the art, the present invention is equally applicable to displays that use diffractive optical elements to couple out image illumination towards the observer's eye.

[0024] The illumination from the projector 100 corresponding to the collimated image is herein shown as being injected into the waveguide at surface 13 in a simple manner that does not replicate the injected image and thus no conjugate image is generated. Thus, the light rays propagating within the waveguide contain "holes", i.e., regions where the image illumination does not reach, and the light rays reaching the observer's eye 40 are non-uniform. Consequently, the intensity distribution detected by the observer is non-uniform and will vary with the different positions of the eye within the "eye movement box" (the allowable viewing positions of the eye) and will depend on the particular field being observed.

[0025] To achieve uniform illumination of the out-coupled light, more advanced in-coupling configurations are typically used, such as Figure 2A and Figure 2B the configurations shown in. Here, the projector 100 provides a larger aperture and is coupled to the LOE via a prism 30 such that the light rays injected into the waveguide and reflected by the lower surface 12 of the waveguide overlap with the light rays directly injected from the projector. This ensures that both the image and its conjugate are fully present within the waveguide, which is referred to as "filling" the waveguide with the image illumination. Of course, this must be true for all fields supported by the waveguide. Figure 2A and Figure 2B show two extreme cases of a typical field of view that is approximately 20° wide in the medium and correspondingly approximately 30° wide in air. It should be noted that a significant portion of the illumination loss for each field (represented by the light rays that terminate on the rear surface of the coupling prism due to falling outside the LOE aperture rather than entering the LOE).

[0026] In principle, advanced projectors can be designed such that each field includes only the light rays that are ultimately coupled into the waveguide. However, these are difficult to design and present many technical complexities (e.g., the aperture in such a system is at an inclined angle to the chief ray and is far from the projector, typically requiring a large projector). In addition, this type of projector must be designed for a specific waveguide, and a "one-size-fits-all" general projector is not possible.

[0027] Now generally referring to certain particularly preferred implementations of the present invention, an optical system is provided that includes a light guide optical element (LOE) 10 formed of a transparent material and having a first major outer surface 11 and a second major outer surface 12 that are parallel to each other for guiding light by internal reflection. The projector 100 is configured to project illumination corresponding to a collimated image from an aperture 101 that exits the aperture along a chief ray that defines the optical axis 102 of the projector and within an angular field around the chief ray. Figure 3B shows a set of light rays parallel to the chief ray, while Figure 3A and Figure 3C show the shallowest angle ray and the steepest angle ray of the angular field, respectively.

[0028] The coupling prism 30 attached to the first major outer surface 11 of the LOE provides at least a portion of an image injection surface 32 that is inclined with respect to the first major outer surface 11 and the second major outer surface 12. In Figures 3A to 3CIn a non-limiting example, the image injection surface 32 is provided in part by the coupling prism 30 and in part by the edge of the LOE 10, and together they are polished to form a continuous surface. The projector 100 is associated with the image injection surface 32 and is oriented such that the chief ray and the angular field around the chief ray are injected through the image injection surface at an angle of incidence with respect to the main outer surface, the angle of incidence being greater than the critical angle of total internal reflection at the main outer surface. In other words, the orientation of the projector and the coupling prism is such that image illumination can propagate within the LOE by total internal reflection at the angles at which they are projected.

[0029] A particular feature of certain preferred implementations of the present invention is that the reflective polarization beam splitter 51 is deployed at the interface between the first main outer surface 11 and the coupling prism 30 parallel to the main outer surface. At least a portion of the illumination from the projector 100 is incident on the beam splitter 51 in a first polarization, and the first polarization is transmitted by the beam splitter from the coupling prism 30 into the LOE 10, while light corresponding to the conjugate image of the collimated image and having a second polarization is incident on the beam splitter from within the LOE and is reflected from the beam splitter so as to propagate within the LOE by total internal reflection. Thus, the beam splitter differentiates the image illumination from the projector 100 (which is allowed to enter the LOE) from the image illumination already within the LOE (which is prevented from escaping), and begins its propagation via total internal reflection along the LOE.

[0030] Various arrangements can be used to effect the polarization adjustment to achieve the above functions. In a particularly preferred subset of examples, a wave plate is deployed in the path of at least a portion of the image illumination to convert the illumination between a first polarization and a second polarization. Figures 3A to 3C An example of this is shown, where the wave plate is implemented as a quarter-wave plate 52 associated with at least a portion of the second main outer surface 12 of the LOE.

[0031] The operation of this implementation is as follows. Light is projected from the projector 100 into the waveguide 10 with p-polarization. (For this example, the option of using p-polarized projected illumination is arbitrarily chosen, but it should be understood that this example could equally well be presented with s-polarized illumination projection, and the p / s polarization names are interchanged throughout). A reflective polarization beam splitter 51 that transmits p-polarized light and reflects s-polarized light is deployed between the coupling prism 30 and the upper surface 11. The optical retarder (quarter-wave plate) 52 is placed on at least a portion of the lower surface 12 and is used to change the polarization of the incident light rays.

[0032] Figure 4Presents graphs of the reflectivity for s - polarization and the transmittance for p - polarization as a function of the angle of incidence, which describe a typical coating of the surface 51, as known in the field of polarization beam splitters. Alternatively, a wire grid polarizer can be used to achieve a suitable effect. Preferably, the retarder 52 will be a quarter - wave plate such that the polarization of the light transmitted back and forth by the retarder will rotate and will be converted from p - polarized light to s - polarized light (or will be converted from s - polarized light to p - polarized light). Due to this structure, the light rays projected by the projector 100 are transmitted through the beam splitter 51 and enter the waveguide. As Figure 3C shown, the light rays incident on the retarder 52 change their polarization and are reflected by the beam splitter 51 if they are incident on the beam splitter 51 a second time. The selective properties of the beam splitter allow a larger portion of the injected image illumination to be coupled into the waveguide, greatly reducing energy loss. In addition, the required size of the projector aperture is significantly smaller than Figure 2A and Figure 2B the required size of the projector aperture in

[0033] The retarder 52 can be implemented in a variety of ways, including but not limited to a crystalline zero - order crystal retarder, a thin - film polycrystalline true zero - order retarder, sub - wavelength structures, and advanced dielectric layers directly coated on the waveguide.

[0034] Optimally, the system is implemented such that the light rays of all fields are reflected only once from the surface 51 before reaching the end of the coupling prism. Otherwise, some light loss will generally occur.

[0035] In this embodiment, the more steeply propagating light rays ( Figure 3C ) may suffer from non - uniform intensity distributions in some cases. This can be alleviated in different ways, for example, by using an embedded mixer element (i.e., a partially reflective surface parallel to the main axis of the waveguide, described below with reference to Figure 8A and Figure 8B ), or by placing closely spaced out - coupling facets in the waveguide. By carefully designing the geometry of the projector aperture and the coupling configuration, the non - uniformity can be alleviated or even eliminated.

[0036] The retarder 52 can be deployed only in the coupling region or can extend across part or all of the waveguide. The retarder can also be used to rotate and mix the polarization along the waveguide and to mitigate any polarization artifacts that may be generated, for example, by the polarization - dependent coupling configuration of this embodiment. The retarder can be located on the outer surface of the waveguide or between the waveguide 10 and an external thin cover plate (not shown), which can be used to enhance the uniformity of the out - coupled illumination.

[0037] In the present and other embodiments described herein, capturing light within the LOE by a beam splitter 51 relatively close to the image injection surface provides an advantage in the design of the image projector 100. Specifically, for optical efficiency, the entrance aperture of the waveguide is preferably imaged by the projector optics (illumination optics plus collimation optics, not shown) to the illumination aperture stop of the projector. In Figure 2A and Figure 2B designs, the effective aperture of the waveguide is located at the end of the coupling prism remote from the image injection surface. In contrast, Figures 3A to 3C designs and the examples that follow herein provide an effective waveguide aperture closer to the image injection surface 32, allowing the use of a general projector design in which the illumination aperture stop is imaged to the projector exit aperture, and generally facilitating the use of a smaller overall size of the projector.

[0038] Figure 5A and Figure 5B show alternative implementations of embodiments of the present invention, in which the image injection surface 32 is provided entirely by the coupling prism 30, such that the projector 100 is placed on top of the waveguide. Such a configuration would significantly ease manufacturing, but would result in a slightly larger aperture. In all other respects, Figure 5A and Figure 5B the structures and operations of the implementations are similar to Figures 3A to 3C the structures and operations.

[0039] Figure 6A and Figure 6B show alternative implementations that do not employ a retarder on the second major surface of the waveguide, but rather a retarder 52 in the form of a half-wave plate that is deployed to overlap a first portion of the aperture 101 and not overlap a second portion of the aperture. In the case shown here, the "first" portion of the aperture is illuminated by that portion of the image injection surface 32 through which light passes through the beam splitter 51. This applies to the case where the projector projects polarized light that is reflected by the beam splitter. As shown, the polarized light reflected by the beam splitter is directly introduced into the LOE in the lower portion of the coupling surface and is thus captured by the beam splitter and propagates along the LOE by total internal reflection, while in the upper portion of the aperture as shown, the half-wave plate 52 converts the illumination of the second polarization to illumination of the first polarization, thereby allowing that portion of the image illumination to be transmitted by the beam splitter and enter the LOE.

[0040] Thus, by way of specific example, in Figure 6A and Figure 6BIn the case where the beam splitter passes p-polarization and reflects s-polarization, the light that is directly injected into the waveguide and does not propagate through the lower part of the image injection surface of the retarder 51 is s-polarized, while the light that propagates through the upper part of the image injection surface of the retarder 51 (preferably used as a half-wave plate here) is injected into the waveguide in p-polarization.

[0041] Obviously, an equivalent effect can be achieved by using a projector that generates the polarization transmitted by the beam splitter and deploying a half-wave plate 52 at the part of the image injection surface 32 where light is directly coupled into the LOE without passing through the beam splitter (the lower part, in the orientation shown here).

[0042] In all embodiments described herein, the beam splitter is described as being at the interface between the first main outer surface 11 and the coupling prism 30 and parallel to the main outer surface. The "interface" for this purpose is functionally defined as the region where light enters the LOE 10 from the coupling prism 30. Most preferably, the beam splitter is deployed coplanar with the first main outer surface 11, typically as a coating applied to one or the other of the opposing surfaces of the coupling prism 30 entering the LOE 10 before bonding, or as a film or other layer sandwiched between the coupling prism 30 and the LOE 10. However, the deployment of a beam splitter embedded within the coupling prism 30 or within the LOE 10 will also be considered "at the interface" as long as it is close enough to the interface to provide the above-described function. In all cases shown, the parallelism of the beam splitter with the main surface of the LOE is necessary to avoid generating ghost images as the image illumination propagates along the LOE.

[0043] The various coupling arrangements described above inherently couple light into the waveguide in a mixed polarization state, i.e., as a superposition of p-polarized light and s-polarized light, such that for a particular field, some regions of the input aperture are composed of p-polarized light while other regions of the input aperture are composed of s-polarized light. Since the embedded (refractive or diffractive) components that couple light out of the waveguide are typically polarization-sensitive, this can produce a striped (non-uniform intensity) image at the output.

[0044] In principle, the embedded elements can be designed and optimized by matching the conditions of the two polarization states to maximize uniformity, but this is usually difficult to achieve; and it will come at the cost of efficiency, color uniformity, etc. Therefore, several alternative methods are proposed below to improve the effect of the mixed polarization illumination coupled into the waveguide.

[0045] As Figures 7A to 7CAs presented, the polarization retarder 201 can be placed inside the waveguide such that it controls the polarization state of the light inside the waveguide. The retarder can be made of a birefringent crystal, a thin polymer layer, or a structured or spatially varying coating or spatially varying grating. Such an element can be embedded inside the waveguide (as described in the co-pending PCT patent application No. PCT / IL2021 / 051143), or if the waveguide and the coupling wedge are produced separately, the retarder can be separately bonded between the waveguide and the coupling wedge. The thickness of the retarder can be set to a preferred thickness. For example, it can be thin such that it will operate as a true quarter-wave plate for the relevant wavelength and taking into account the angle of incidence of all fields in the field of view (FOV), such that the s-polarized light and p-polarized light transmitted by the retarder will be converted to (approximate) circularly polarized (but with opposite handedness).

[0046] In an alternative but conceptually related implementation, the projector 100 can be configured to generate circularly polarized image illumination, and the polarization beam splitter 51 can accordingly be implemented as a circular polarization beam splitter. In this way, the light coupled into the waveguide will be right-circularly polarized or left-circularly polarized, and the uniformity of the output light will be significantly improved.

[0047] Alternatively, the retarder can be made "thick", defined by where d is the thickness of the retarder, Δλ is the spectral bandwidth of each transmitted color, and Δn = |n e - n o |, where n e and n o are the extraordinary refractive index and the ordinary refractive index of the retarder. Typically, d ~ 0.1 mm - 1 mm is sufficient for the retarder to provide a "depolarizing" effect. Specifically, different wavelengths within a given color spectral bandwidth are rotated to different polarization states, and the superposition of all wavelengths effectively appears as unpolarized light.

[0048] Due to the different optical paths through the retarder that can generate ghost images, a thick retarder may cause unwanted artifacts in the Figure 7B and Figure 7C configurations. As Figure 7A shown, this problem can be solved if the retarder is placed perpendicular to the waveguide with sufficient precision. In this case, the angular orientation of all the light rays propagating through the retarder is maintained, and no ghost images are expected.

[0049] If the projector outputs image illumination in a polarization state that is not orthogonal to the waveguide, i.e., not pure s-polarized light or pure p-polarized light according to the waveguide axis, but a linear superposition of both, then the polarization of each wavelength will rotate at each reflection of TIR on the main surface of the waveguide. This will effectively have a similar effect to the thick retarder in FIG. 7. As shown in patent WO2021105978A1, this mixing of the coating can be further enhanced by coating the main surface of the waveguide with a special coating.

[0050] Another method could be to place a partially reflective layer 202 in the middle of the waveguide and parallel to the main outer surface (as disclosed in PCT patent application publication No. WO 2021 / 079372), which will mix the light. An example of such a structure is shown in Figure 8A and Figure 8B According to this option, the light in each field will be uniform throughout the waveguide. However, the ratio of p-polarized light and s-polarized light may still be different between one field and another. This effect needs to be considered when designing the characteristics of diffractive or refractive elements embedded inside the waveguide.

[0051] The various implementations of the present invention described herein are applicable to a wide range of environments and employ any type of waveguide and any type of projector. For example, the projector 100 can employ any suitable image generation technology, including but not limited to liquid crystal transmissive or reflective (LCOS) projectors, scanned laser projectors, or DLP projectors, all of which employ any suitable collimating optics.

[0052] It should be understood that the above description is only intended to be illustrative, and many other implementations are possible within the scope of the present invention as defined by the appended claims.

Claims

1. An optical system, comprising: (a) A light guide optical element (LOE) formed of a transparent material and having a first major outer surface and a second major outer surface parallel to each other for guiding light by internal reflection; (b) A projector configured to project illumination corresponding to a collimated image from an aperture, the illumination leaving the aperture with a chief ray defining an optical axis of the projector and within an angular field around the chief ray; (c) A coupling prism attached to the first major outer surface of the LOE, the coupling prism providing a portion of an image injection surface that is inclined with respect to the major outer surface, the projector being associated with the image injection surface and oriented such that the chief ray and the angular field around the chief ray are injected through the image injection surface at an incident angle with respect to the major outer surface that is greater than a critical angle of internal reflection at the major outer surface; and (d) A reflective polarization beam splitter deployed at an interface between the major outer surface and the coupling prism parallel to the major outer surface, a portion of the illumination being incident on the beam splitter with a first polarization and being transmitted by the beam splitter from the coupling prism into the LOE, light corresponding to a conjugate image of the collimated image and having a second polarization that is incident on the beam splitter from within the LOE being reflected by the beam splitter for propagation within the LOE by internal reflection.

2. The optical system according to claim 1, further comprising: A wave plate deployed in a path of a portion of the illumination to convert the illumination between the first polarization and the second polarization.

3. The optical system according to claim 2, wherein, the wave plate is a quarter-wave plate associated with at least a portion of the second major outer surface of the LOE.

4. The optical system according to claim 2, wherein, the wave plate is a half-wave plate, the half-wave plate being deployed to overlap a first portion of the aperture and not to overlap a second portion of the aperture.

5. The optical system according to claim 4, wherein, the first portion of the aperture projects illumination through a portion of the image injection surface through which light enters the LOE without passing through the beam splitter.

6. The optical system according to claim 4, wherein, the projector is configured to project illumination with the second polarization, wherein the first portion of the aperture projects illumination through a portion of the image injection surface through which light passes through the beam splitter, and the half-wave plate converts the illumination with the second polarization to illumination with the first polarization.

7. The optical system according to claim 1, wherein, the image injection surface is provided partially by the coupling prism and partially by a surface of the LOE.

Images